1. Field of the Invention
The present invention relates to a silicon wafer cleaning method and more particularly to a silicon wafer cleaning method for cleaning the silicon wafer with the surface structure of a high-temperature annealed silicon wafer maintained.
2. Description of the Related Art
The surface of a silicon wafer flattened up to atomic level has a so-called step and terrace structure in which layers of silicon atoms are formed in step-like formation. As shown in FIG. 1, this step and terrace structure is comprised of a terrace face 11 which is a sliced face of the wafer, and a step face 12 in which minute steps on atomic level are formed with respect to the terrace face 11.
For example, by annealing at high temperatures of about 1000 to 1200° C. under an environment of hydrogen gas or the like, usually, a step equal to one to three layers of silicon atoms or a step and terrace structure having atomic level step on the surface is formed. This is formed because silicon atoms on the wafer surface are rearranged for stabilization.
The terrace width of the aforementioned step and terrace structure is increased by decreasing an off angle of silicon crystal, so that the annealed silicon wafer surface comes to have a structure flattened up to atomic level.
After the surface is flattened by the aforementioned hydrogen annealing or the like, the silicon wafer is sent to cleaning process of silicon wafer manufacturing process in order to remove particles and metallic foreign matter from the surface of the wafer.
Conventionally, in this cleaning process, RCA cleaning has been employed as a general method.
In the RCA cleaning, SC-1 cleaning liquid composed of ammonia: hydrogen peroxide: pure water=1:1 to 2:5 to 7 (volume mixing ratio) is used to remove, for example, mainly organic pollutant and adhering particles as a cleaning liquid. Further, to remove mainly silicon oxide layer and surface metallic foreign matter, thin hydrofluoric acid (DHF liquid) composed of HF: pure water=1:99 is used, and further to remove mainly surface metallic foreign matter, SC-2 cleaning liquid composed of muriatic acid: hydrogen peroxide: pure water=1:1 to 2:5 to 7 (volume mixing ratio) is used.
However, in the RCA (SC-1) cleaning, silicon on the wafer surface is subjected to anisotropic etching with ammonia and the step and terrace structure formed by annealing with hydrogen gas or the like is vanished, so that the surface roughness (micro roughness) is worsened.
As another cleaning method, a cleaning method using mixing solution of hydrogen fluoride and ozonized water has been disclosed in, for example, Japanese Patent Publication No.10-340876.
According to this method, oxide film is formed on the wafer surface with ozonized water by injecting hydrogen fluoride and ozonized water to the wafer surface at the same time. This method enables particles to be removed without exposing silicon layer in a condition that the contact angle between the wafer surface and hydrogen fluoride is small and the surface is likely to be wet.
However, when hydrogen fluoride and ozonized water are used in mixture, oxide film is formed unequally within a plane on the wafer surface. Thus, this method is incapable of suppressing deterioration of the surface roughness (micro roughness).
With recently intensified integration of semiconductor circuit, miniaturization of a formation device has been demanded and for example, gate oxide film thickness of about 3 nm has been demanded in processing on the level of 0.13 nm in terms of line width.
Corresponding to such a thinned thickness of the oxide film, the fine roughness of the wafer, that is, micro roughness has been demanded to be further reduced in size.
Usually, a silicon wafer is sliced so as to have its crystal orientation of Si (100) plane or Si (111) plane when it is cut off from a single crystal ingot. Particularly, in case of the Si (100) plane, flattening on atomic level is difficult and therefore, the micro roughness has been demanded to be reduced as much as possible.
To remove particles and metallic foreign matter after annealing or obtain a wafer having a clean surface, wafer cleaning process is an indispensable process although the surface roughness (micro roughness) is worsened as described above.
Therefore, technology capable of cleaning a wafer with its step and terrace structure on the wafer surface after annealing maintained, despite any crystal orientation of the wafer surface has been demanded.